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The molecular structures of three putative boundary lubricants, (A) superficial zone protein (SZP), (B) hyaluronic acid, and (C) surface active phospholipids (e.g., phosphatidylcholine) are represented. SZP is a 345 kDa proteoglycan and a product of the gene prg4. Hyaluronic acid is a disaccharide polymer consisting of repeating units of glucuronic acid and N-acetylglucosamine. Phosphatidylcholine can be composed of a variety of fatty acid structures.
